To get the iso's for testing you have to become a tester first. The iso's are not released public.
If you have a account on that Community site you have to contact Clem and he will add you to the testers group.

The current iso's of LMDE are going to be rejected due some DNS resolution problems, so testing these any further won't bring much. When the new iso's will be up for testing i don't know yet.